The invention relates to a riser guide for use on a floating offshore platform. The platform comprises a topsides and a substructure having a lower pontoon, and at least one riser extends from a subsea location to the topsides.
The invention also relates to a method for installing riser guides on a floating offshore platform, and a method for removing riser guides on a floating offshore platform.
In offshore hydrocarbon production, hydrocarbons flow from a subterranean formation into a well, and up to the sea bed. From the sea bed the hydrocarbons flow to a platform via risers. Risers can also be used for water or gas injection, in order to maintain the pressure in the reservoir, or for supplying pressurised hydraulic oil and electric signals for energising and controlling subsea equipment which is used in the hydrocarbon production.
In shallow and medium depth waters fixed platforms resting on the sea bed are used. In deep seas a structure resting on the sea bed would be too large, and therefore floating platforms are used. Due to the motion of the sea, a floating platform is almost always moving. The risers may be stiff steel risers, which are prone to overstressing due to the motions of the floating platform. In order to overcome the problem of the moving platforms, flexible risers may be used. Flexible risers are, however, more expensive than stiff risers.
Irrespectively of what type of risers are used, they must to some extent be laterally guided. Typically riser guides will be located at the pontoon, The riser guides may include pads which are located close to or in abutment with the riser, for laterally guiding the riser during the movement of the platform.
WO 00/58598 discloses a riser guide system comprising a framework which is located around the riser and secured to the platform. Rollers, in the illustrated embodiment having a number of four, are located in the framework, close to or in abutment with the riser, for laterally guiding the riser.
Usually riser guides will be installed subsea, maybe at the pontoon 20-30 meter below the sea surface. This installation may be carried out by divers or an ROV (remote operated vehicle). This can be dangerous and problematic, and it is therefore desirable to find other ways to do this installation, without divers or an ROV.
Riser guides will after some time be worn, and they must therefore be replaced. Divers or an ROV may be used, but again this can be dangerous and problematic, and it is desirable to find other ways of replacing the riser guides.
An object of the invention is to provide riser guides which can be installed and replaced from the topsides. A further object is to provide a method for installing riser guides on a floating offshore platform, and a method for removing riser guides on a floating offshore platform, which methods shall be carried out from the topsides. A particular object is that the invention shall be suitable for stiff risers.
The objects are achieved by a riser guide and methods according to the claims.
The invention thus relates to a riser guide for use on a floating offshore platform, the platform comprises a topsides and a substructure having a lower pontoon, at least one riser extends from a subsea location to the topsides, and a number of riser guides are located in a guide housing which is secured to the platform and have a through-going opening for the riser.
Each riser guide comprises a support arm essentially radially arranged in the guide housing, having an outer end facing away from the opening in the guide housing, the outer end of the support arm is rotatably mounted about a horizontal axis in the guide housing, and an inner end facing the riser, the inner end is tiltable between a lower and an upper position. The riser guide also comprises a roller rotatably mounted about a horizontal axis in the inner end of the support arm, the roller is tiltable between a lower position close to or in abutment with the riser, for laterally guiding the riser during movement of the platform in the sea, and an upper position away from the riser. Further the riser comprises a wedge provided with a connection for lifting gear, a mechanical link extends between the wedge and a connection on the support arm radially inwards from the horizontal axis of the support arm, the wedge is vertically movable between a lower position between the outer end of the support arm and the guide housing and an upper position above the support arm, in the upper position the wedge has lifted the inner end of the support arm with the roller to the upper position by means of the mechanical link.
When installing riser guides according to the invention on a floating offshore platform, the following steps are carried out:
a) placing a guide housing with a through-going opening around the riser at the topsides, a number of riser guides are located in the guide housing,
b) connecting lifting gear to the wedges and tensioning the lifting gear, causing the rollers to move to their upper position,
c) by means of the lifting gear lowering the guide housing into a primary guide secured to the platform, and
d) slackening the lifting gear, thereby lowering the wedges and the rollers to their lower positions.
The lifting gear can be operated from the topsides, and the riser guides are thereby installed from the topsides.
When removing riser guides according to the invention on a floating offshore platform, the following steps are carried out:
a) tensioning the lifting gear, thereby lifting the wedges and the rollers to their upper positions,
b) by means of the lifting gear lifting the guide housing out of the primary guide,
c) lifting the guide housing up to the topsides, and
d) removing the riser guides from the guide housing.
The lifting gear can be operated from the topsides, and the riser guides are thereby removed from the topsides. A guide housing with new or repaired rollers can then be installed as discussed above. A replacement of the riser guides from the topsides has thereby been carried out.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.